1 Neonatal Unit, University Hospitals of Leicester, UK
2 Department of Epidemiology, University of Leicester, UK

Background: Despite a falling annual birth rate and the improved
survival of preterm infants below 28 weeks gestation there is a trend towards
increasing numbers of preterm births. As such accurate, up to date normative
data on gestational age specific birth weight are vital. However, the most
commonly used weight standards for infants born before 32 weeks are some twenty
years out of date and we would question methodological issues due to inclusion
of data from several sources (primarily hospital based, thus unrepresentative)
spanning over 20 year period.

Aim: To produce reliable birth weight for gestational age centile charts
for a geographically defined population representative of the UK.

Methods: The study was based in the Trent Health Region, UK, which has a
population of 4.6 million and approximately 55,000 births. Data on all singleton
live births of between 24 and 32 weeks completed gestation born between 1998 and
2001 to Trent resident mothers of European ethnicity were included. This was a
total cohort of newborn infants from CESDI and Trent Neonatal Survey dataset.
Newborns with congenital malformation were excluded. Birth weight centiles were
estimated using quantile regression.

Results: There were 2510 infants included in the study, of which 1331
(53%) were males. The median values (range) for birth weight and gestation were
1.37 kg (0.32 to 2.87) and 30 weeks (24 to 32) respectively. In both boys and
girls and at each gestation there was very strong evidence of a non-linear
relationship between birth weight and gestation at birth. When compared to
existing charts (1) major differences were noted in the lower centiles for birth
weight. Our 3rd and 10th percentiles were significantly lower than estimates
from Freeman et al. This has implication in the classification of preterm
infants as small for gestational age.

Conclusion: Easy to use population based birth weight for gestational age
centile charts facilitate decision making for clinicians and parents. It is
important that these charts are regularly updated to reflect changes in the
prevalence of preterm infants, specifically, to identify gestation specific
intrauterine growth retardation.